High performance passive and active optical lens systems need very high quality optical elements in order to obtain the required degree of performance. Optical lens systems are composed of multiple individual lens elements designed to achieve the necessary performance over a specified field of view and specified spectral band. The three main considerations in the design of an optical system are cost, weight, and performance.
To control the costs of optical systems, the optical systems are generally designed to contain only spherical surfaces. The use of aspherical surfaces in optical systems can reduce the number of elements needed to achieve the required performance. However, lens elements having aspherical surfaces are significantly more expensive to manufacture than lens elements having only spherical surfaces. Therefore, the decision to use aspherical surfaces in place of spherical surfaces in a particular optical lens system depends upon whether the weight of the optical system is a more critical factor than the cost of the system or whether the cost of the system is a more critical factor than the weight of the optical system.
Examples of optical systems that can require very high quality optical systems are passive and active infrared sensors and infrared camera systems. The number of optical materials that are transmissive at infrared wavelengths is limited. Most infrared lens elements are made from the four materials, germanium, silicon, zinc sulfide, and zinc selenide, all of which are dispersive. The dispersive characteristic of these materials results in an optical system that contains more elements than the optical system would if less dispersive materials were available. The additional elements are necessary to correct chromatic aberration, which is a consequence of dispersion and causes light to have different focal points depending upon wavelength.
The image produced by an optical system has other aberrations that take many forms, such as spherical aberration, spherochromatism, and coma. Spherical aberration results when spherical lens surfaces are used and causes light striking nearer the periphery of the lens to be focused closer to the lens while light striking near the center is focused farther away from the lens. Spherochromatism (or spherochromatic aberration) is a type of spherical aberration in which the focal points of light rays vary with the wavelength of the light. Coma is an aberration that distorts images formed by off-axis light rays that do not strike the lens at its center.
Conventional optical systems therefore require additional lens elements to correct for aberrations, thereby adding cost, weight, size, and complexity to the lens system in order to correct for the aberrations introduced into the lens system. The curvatures of the spherical surfaces of the elements are designed to reduce both the spherical and chromatic aberrations to an acceptable amount. However, without corrective lenses, conventional optical systems would be limited by chromatic aberration to low speeds or low focal ratios.
One alternative to adding lens elements to correct for aberrations is to use diffractive surfaces, such as one of many types of computer-generated Fresnel zone plates manufactured on at least one side of a lens element. A high efficiency binary approximation of a Kinoform-type Fresnel zone plate, called a binary grating surface, has been disclosed for use in infrared systems by Swanson and Veldkamp in "Diffractive optical elements for use in infrared systems," Optical Engineering, Vol.28 (6), pp. 605-608 (June 1989). The use of the diffractive surface can be used with a spherical or conventional lens. When used with a spherical surface, the spherical surface provides focusing and the diffractive surface corrects for as much of the spherical aberration as possible. However, the use of a diffractive surface to correct for spherical aberration results in a significant amount of spherochromatism. In addition, because low speeds or focal ratios are required to limit this chromatic aberration, the usable optical speed is significantly limited.
In an attempt to solve these shortcomings, in U.S. Pat. No. 5,044,706, Chen proposed an optical element having aspherical and binary grating optical surfaces. In embodiment of Chen, the optical element is a positive meniscus optical element made of germanium having a useful spectral bandpass in the infrared wavelength region. A telescope system incorporating two of the optical elements has an aspherical surface and a diffractive surface. In another of Chen's embodiments, an objective element having a convex aspherical surface and a concave binary grating surface is used in the telescope. The necessity of using two of the optical elements each having an aspherical surface and a diffractive surface results in an optical system that uses fewer lens elements than previous optical systems. However, as can be appreciated by one of ordinary skill in the art, the resulting optical systems are very expensive. The fabrication of a lens element having an aspherical surface and a diffractive surface is very difficult and expensive. In addition, the alignment of a system using one or more of the combined lens elements is critical to the performance of the optical system.
Accordingly, there is a need for a low cost objective lens system that has improved aberration correction and that provides axial color and spherochromatism correction so that chromatic aberration is not propagated through the optical system.